Spinal fusion is a common surgical procedure used to correct numerous disease states including degenerative disorders, trauma, instability, and deformity. An often used method of fusion entails the use of bone screws placed through various sections of the vertebral body including the body, pedicle, facets, lamina, lateral masses, and/or transverse processes. These screws are then linked rigidly with a rod, plate or other fixation device to immobilize the vertebral segments.
Typical spinal bone screws include a shank portion connected to an enlarged head portion also called a receiver or tulip. The shank portion includes external threads for insertion into bone. Self-tapping screws or screws that require pre-tapping are available. The head portion of the bone screw includes a U-shaped channel configured for receiving a stabilization rod. A rod is inserted into the channel and a set screw is threaded into the channel to secure one end of the rod to the screw. The other end of the rod is connected to a second bone screw inserted into an adjacent vertebral body. Thereby, the rod spans an intervertebral disc space to stabilize the motion segment. Multiple rods and screws can be employed for multi-level stabilization. The receiver portion of the screw is typically connected in a manner that permits angulation of the receiver relative to the shank portion. Due to the variation in a patient's anatomy and differences in screw placement technique, polyaxial or multi-axial bone screws allow for a variation in the angulation of the receiver portion relative to the shank portion in order to allow the receiver portion to more closely align for receiving a fixation rod within the channel of the receiver portion. Fixed receiver screws may also be employed.
To insert a bone screw, an insertion instrument is attached to the receiver portion of the screw and the insertion instrument and the screw are inserted through an incision. The incision may be small as in minimally invasive surgical procedures or in mini-open procedures or the incision may be large as in open surgical procedures. The clinician places the bone screw in the most optimum position with or without over-the-guidewire placement of a cannulated screw while checking correct alignment with the bony anatomy via fluoroscopic observation. Much skill is required and the clinician relies heavily on the proper functioning of instrumentation in the most difficult anatomical situations.
The bone screw insertion instrument is typically an elongated tower that connects to the screw at the distal end. If a polyaxial bone screw is employed with the insertion instrument, the insertion instrument must be able to lock the angulation of the head relative to the shank so that the screw is firmly inserted into the bone. Sometimes forces during the placement of the screw are so great that the insertion instrument is knocked off the screw. Typically, the insertion instrument includes two oppositely placed prongs that mate with recesses formed on the outer surface of the head. Sometimes, the prongs of the insertion instrument get hooked on adjacent anatomy and result in the insertion instrument detaching from the screw. Furthermore, because space is at a premium in the spinal anatomy, the head portion is typically designed to be as small as possible so that it does not protrude from the skin or impinge on adjacent anatomy after surgery. Although, the size of the implants is a major driving force in spinal bone screw design, the strength of spinal implants and instruments is extremely important. Generally, the contact area of the prongs of the insertion instrument with the recesses on the receiver as well as the depth of the recesses are as small as possible because of competing size and strength design considerations. As a result, the insertion instrument sometimes dislodges from the screw. Typically, the prongs of an insertion instrument are covered to close the prongs onto the receiver. Because the distal-most portion remains uncovered in order to navigate the insertion between crowded anatomy, the prongs may splay slightly outwardly especially under torsional forces undergone during screw placement which may result in the insertion instrument slipping off the screw. The receiver is connected to the distal end of the insertion instrument and the instrument is manipulated at the proximal end which results in amplification of the input force at the proximal end and a leveraged output force at the distal end of the instrument. Such angulation can result in the screw disconnecting from the insertion instrument. Other forces including torsional forces and pulling of the instrument may also dislodge the screw from the instrument. Precious time is lost when the clinician must re-attach the screw to the instrument and continue with the procedure. For all of these reasons, a strong need exists for an improved screw and instrument system that strengthens the connective interface between the screw and the insertion instrument under the demands of size and use restrictions unique to spinal surgery. The present invention provides such an improved bone screw and insertion instrument system.